In order to measure the moisture content of lumber during drying, it is impractical to weight every board in the dryer and electric meters are not accurate enough. Therefore, we used the sample board method to estimate the moisture of the lumber in the kiln. This method uses short, carefully chosen sample boards sawn from larger pieces of lumber. These sample boards are weighted periodically and their moisture contents are calculated. Generally, because the wettest lumber in the kiln has the highest risk of degrade development, the sample boards should represent the wettest lumber in the dryer; this means the lumber that is the most recently cut, is the widest and thickest, contains the heartwood, and is quartersawn.

The precise procedures to prepare sample boards are:

Select lumber to be used.

Cut a 30-inch (76 cm) sample board and then two 1-inch (2.5 cm) moisture sections from the sample board, avoiding areas near knots and areas closer than 12-inches (30 cm) from the ends of the lumber. See figure.

Number the sections and the sample board.

Immediately weigh the sections. An accuracy of about 0.002 pounds (0.03) ounces or 1 gram) is required. It is important that they not lose weight after cutting and before weighing. Record weight on the section with a marker pen.

Weigh the sample board. An accuracy of about 0.1 pounds (1.5 ounces of 45 grams) is required. Record the weight on the sample.

End coat the sample board.

Place sample board in the lumber stack in a location where it will dry at the same rate as the rest of the lumber in the dryer.

Place sections in an oven at 215 F (100 C). (Note: Home kitchen microwave ovens can be used only under low power and with a tray to rotate the samples.)

After the sections are dry – usually 18 to 36 hours depending on the oven – reweigh and obtain the oven-dry weight. (Microwave ovens will require 20 minutes.) Always weigh the samples once, place them back into the oven for a short while, and then reweigh again – if oven dry, the two weights will be the same.

Calculate the moisture content of each section separately and then average the MC of the two sections to obtain the estimated MC of the sample board when it was cut (step 2).

Occasionally it is necessary to cut new sample boards when the lumber is at 20% MC to obtain more accurate MC values (that is, to make a mid-course correction). To do this, cut one new moisture section 6-inches from the end of the sample board. Then follow steps 3 through 13, except only one section is used. The "wet weights" in steps 10 and 11 are the weights determined after the sample and new section are sawn.

Questions entered in the Knowledge Base Article comment form will not generate responses! A list of WOODWEB Forums can be found at WOODWEB's Site Map.

When you post your question at the Forum, be sure to include references to the Knowledge Base article that inspired your question. The more information you provide with your question, the better your chances are of receiving responses.

The editors, writers, and staff at WOODWEB try to promote safe practices.
What is safe for one woodworker under certain conditions may not be safe
for others in different circumstances. Readers should undertake the use
of materials and methods discussed at WOODWEB after considerate evaluation,
and at their own risk.